The Genetics of Emotional Ontogeny (GEO) project entails two fine-grained longitudinal studies of behavioral development in twins. Genetic and environmental underpinnings of the timing of behavioral development are investigated. Previous twin studies have frequently focused on the level of expression of emotional, social, and cognitive behaviors, but seldomly on the timing of behavioral development. Biobehavioral scientists seem confident that dynamic gene action affects behavioral change throughout the lifespan, yet a genetic basis for the timing of such changes has not been convincingly demonstrated. The primary study, Project A, is a longitudinal twin study with biweekly laboratory to assessment, supplemented by maternal report. Thus, even short intrapair differences in the appearance of a newly organized affective-behavioral patterns, such as social smiling, can be detected. During the first year, the initial appearance and increasing organization of primary emotions (e.g., joy, anger, and fear) is studied. Later, the emergence of more complex social emotional reactions (e.g., empathy, pride, shame, and guilt) is studied. The three-pronged assessment approach includes (1) weekly maternal reports; (2) biweekly elicitation of emotional reactions during laboratory visits; and (3) brief interviews with mothers. It is unlikely that the rate of emotional development is independent of changes in other domains, so fine-grained ordinal scales to assess thc timing of maturation in certain physical, motoric, and cognitive domains is included. These include neonatal reflexes, motoric skills, spatial search ability, lateralization, cognitive sophistication of social play, and ability to represent the self. Project B is designed to increase the sample size for key analyses economically. It involves recruiting larger numbers of twins and assessing them with an abbreviated protocol that can be implemented by regular visits to the home in a research van, questionnaires, and telephone interviews. This extended study focuses only on certain key time periods.